1. Field of the Invention
The present invention relates to a method and apparatus for producing a magnetic recording medium.
2. Description of the Related Art
As a recording density of a magnetic read/write equipment has been increased year by year, it is highly desired to provide a magnetic recording medium which has excellent read/write characteristics in a short wavelength range. Today, a coating type of magnetic recording medium in which magnetic powder is coated on a substrate is mainly used, and its properties are being improved to satisfy the above desire. However, the improvement of the property is nearing its limit.
One of magnetic recording media which can exceed this limit is a thin film magnetic recording medium. The thin film magnetic recording medium is produced by a vacuum deposition method, a sputtering method or a plating method and has excellent read/write characteristic in a short wavelength range. Examples of magnetic materials used in the thin film magnetic recording medium are Co, Co--Ni, Co--Ni--P, Co--O, Co--Ni--O, Co--Fe--O, Co--Ni--Fe--O, Co--Cr, Co--Ni--Cr, and the like. Among them, a layer of a partial oxide such as Co--O and Co--Ni--O are the most suitable in view of the practical application in a magnetic tape, and a deposition tape comprising a Co--Ni--O magnetic layer is practically used as a Hi-8 type of video tape.
One embodiment of the production method of a deposition tape will be explained by making reference to FIG. 1, which schematically illustrates an example of an inner structure of a continuous vacuum deposition apparatus.
A substrate 1 made of, for example, a polymer film is unwound from a supply roll 3, travels around a peripheral surface of a cylindrical drum 2 in a direction of an arrow, and finally wound on a take-up roll 4.
A vapor of a raw magnetic material such as metal cobalt or a cobalt alloy is evaporated from an evaporation source 5 and deposited on the substrate 1 to form a magnetic layer on the substrate 1. As the evaporation source 5, an electron beam evaporation source is preferred, since it can evaporate a metal having a high melting point such as cobalt, at a high evaporation rate.
Between the evaporation source 5 and the cylindrical drum 2, there are provided two shielding plates 61 and 62, which prevent excessive deposition of the atom vapor on the substrate 1 and define a range of an incident angle of the atom vapor on the substrate 1.
The incident angle is defined as an angle between an incident direction of the atom vapor and a line normal to the substrate 1. The shielding plate 61 defines an initial incident angle .phi..sub.i of the atom vapor in relation to the substrate, while the shielding plate 62 defines a final incident angle .phi..sub.f of the atom vapor in relation to the substrate. The incident angle is one of the important factors which define the magnetic characteristics of the magnetic layer. In the practical method, the apparatus is so designed that the incident angle does not change during a long deposition period (cf. Japanese Patent KOKAI Publication No. 282479/1990).
The continuous deposition apparatus of FIG. 1 has an oxygen gas-supplying nozzle from which an oxygen gas is supplied into a vacuum chamber. The interior of the continuous deposition apparatus is divided into two sub-rooms A and B, which have exhaust outlets 8 and 9, respectively, for evacuating the respective sub-rooms. The outlets 8 and 9 are connected to respective evacuation pumps (not shown). The exhaust outlets may be provided on the side wall of the apparatus, and two or more exhaust outlets may be provided on each sub-room.
An atmosphere during the vapor deposition is another one of important factors which define the magnetic characteristics of the magnetic layer, and the positions of the exhaust outlets can be specially designed (cf. Japanese Patent Publication No. 72014/1993).
In the Co--O or Co--Ni--O magnetic layer formed by the above method, an axis of easy magnetization inclines at a certain angle from a normal line of the layer plane. That is, the axis of easy magnetization is not on the normal line of the layer plane or in the layer plane but inclined from the normal line in a plane including an incident direction of the atom vapor onto the substrate.
As explained above, the magnetic characteristics of the magnetic layer of the partial oxide are greatly influenced by the incident angle of the atom vapor on the substrate, the amount of oxygen gas to be supplied during the vapor deposition, a method for supplying the oxygen gas, and the atmosphere during the vapor deposition. Accordingly, the read/write characteristics of the magnetic recording medium are also influenced by those factors.